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Collinear Resonance Ionization Spectroscopy of potassium isotopes: crossing N=32

Boek - Dissertatie

Understanding the properties of unstable nuclei has been a long-standing problem. In particular, the emergence and disappearance of so called magic numbers has attracted significant attention in recent years, both from the theoretical and experimental side. This thesis aims to contribute to the field of nuclear physics through the measurement of the ground state nuclear magnetic moments and the changes in the mean-square charge radii of neutron-rich potassium (K) isotopes (Z = 19). The K isotopes are within the reach of the state-of-the-art theoretical calculations and therefore serve as an excellent laboratory for testing new developments. Furthermore, in the Ca region new subshell closures are expected to emerge at neutron numbers N = 32 and N = 34. The study of 52K, which has N = 33 neutrons, allows for the investigation of the magic nature of N = 32 by looking at the evolution of the size of K isotopes leading up to and across N = 32. Furthermore, from the nuclear magnetic moment, the ground state configuration of this isotope can be extracted and compared to the empirical and theoretical estimates, further testing the magicity of N = 32 and Z = 20. The Collinear Resonance Ionization Spectroscopy technique was used for performing the measurements discussed in this thesis. First, the optimization of a resonance ionization scheme which provides high spectral resolution and high efficiency is presented. Next, developments towards improved precision are discussed. Both of these were essential to ensure the reliability of the results obtained with the CRIS technique, previously only used to study heavier systems. A simple decay station was assembled which could detect the decay of the short-lived K isotopes, while being insensitive to the non-resonantly ionized stable contamination. This way, the selectivity of the technique can be drastically improved. The aforementioned developments lead to the successful measurement of the hyperfine structure of 52K. The newly determined nuclear spin of this isotope supports the previous tentative assignment of I = 2. The experimental nuclear magnetic moment is well reproduced by shell model calculations using the SDPF-U interaction. The rather pure ground state wavefunction is dominated by the coupling between the proton in the d3/2 and the neutron in the p1/2 orbital. The change in the mean-square charge radius fits into the smoothly increasing trend of radii in the K chain above N = 28 and doesn't feature a kink, normally expected at shell closures. Furthermore, the charge radii were compared to state-of-the-art coupled cluster calculations using a new NNLO interaction derived from chiral effective theory. The disagreement between theoretical predictions and experimental values in the vicinity of N = 32 might indicate that the nuclear structure of these isotopes is more complex than expected.